WebmasterQuery

There's a few websites that will convert docx to doc for you.  I wont post them as a Google search will give you a selection.  

Another way to do it would be using Google docs.  Simply upload the file in question, it'll open in Google docs no problem and then copy and paste the document in to 2003.  There's probably a way to export the document as whatever format you want also, but I haven't used docs in a while.